The present invention relates generally to an automatic transmission, and more particularly to a process of abnormality handling control at occurrence of a failure in engaging elements provided in the automatic transmission.
Japanese Patent Application Publication No. 2000-240785, henceforth referred to as “JP2000-240785”, shows a process of abnormality handling control for automatic transmission, which identifies an abnormal part or abnormal engaging element based on comparison between a selected gear and an actual transmission gear ratio, and shifts into an escape gear or backup gear in accordance with a state of the identified abnormal part.
On the other hand, Japanese Patent Application Publication No. 2003-49937, henceforth referred to as “JP2003-49937”, shows a process of escaping from an undesired interlock state of an automatic transmission. In order to detect an abnormality due to stiction of a valve for regulating a hydraulic pressure supplied to each frictional engaging element, the automatic transmission includes a signal-pressure-operated valve which operates in response to occurrence of at least an abnormality in the hydraulic pressure supplied to one of the frictional engaging elements. The automatic transmission also includes a hydraulic switch provided in downstream of the signal-pressure-operated valve to detect a change in the hydraulic pressure due to operation of the signal-pressure-operated valve. When the hydraulic switch detects a change in the hydraulic pressure, the hydraulic pressure of each engaging element is relieved in order to bring the automatic transmission into a neutral state and thereby to escape from the interlock state.